It is well known to use ultrasonic waves to obtain information on the surfaces of an object, for example a pole or pipe. Typically, ultrasonic pulses are transmitted towards the object, the reflected pulses are received and the travel times of the pulses are recorded. Any differences in travel times (“time of flight”) of the pulses are indicative of differences in the relative height of the surface and hence of the wall thickness of the object. An example of this known technique is disclosed in U.S. Pat. No. 3,930,404. This type of method has the disadvantage that the measurements are extremely local: the surface is only probed at the points of impact (and reflection) of the ultrasonic waves. To obtain information on the remaining part of the surface, the ultrasonic transducers have to be moved relative to the object, and the travel times have to be measured in many other surface points.
International Patent Application WO 2006/000668 discloses a method for the dimensional characterisation of a cylindrical object. Probes facing measuring points on the surface of the object transmit pulsed ultrasonic waves to these measuring points and receive the reflected pulsed waves. To obtain dimensional information on the entire surface of the object without requiring an infinite number of measuring points, a characteristic curve of the surface is obtained by interpolation. Although information on other surface points can be obtained through interpolation, the level of detail of this information is limited by the number of measuring points. This known method therefore also has the fundamental disadvantage that surface information is obtained from only a small number of measuring points, and that a relative large number of probes and/or a movement of the probes relative to the object are necessary to obtain detailed dimensional information.
U.S. Pat. No. 5,965,818 discloses a method using ultrasonic Lamb waves to measure reduction of wall thickness due to localised corrosion at pipe supports. Two transducers are used to make a Lamb wave travel along the pipe wall in the circumferential direction. By comparing measured time-of-flight data the change in time-of-flight due to corrosion can be quantified. Corrosion is only detected in the circumferential direction of the pipe, not in the longitudinal direction.
U.S. Pat. No. 5,767,408 discloses a method of obtaining near-surface characteristics of a material by generating a broadband ultrasonic Rayleigh wave including a plurality of components. Velocities of selected components of the Rayleigh wave are determined at the selected frequencies. However, the wave trajectories are very limited and therefore the information concerning the surface tested is also limited.
British Patent Application GB 2 383 413 discloses a system for detecting defects in rails by using the velocity, attenuation, scattering, resonance and frequency absorbing properties of acoustic surface waves. This known system is quite specific to railway lines and uses electro-acoustic techniques to test rails in their longitudinal direction.
Japanese Patent Application JP 57 187609 discloses another conventional method of measuring wall thickness by using surface waves having an increasing frequency and detecting the travel times. The surface waves have limited trajectories in one direction only and therefore provide only limited information.